A glance at the picture of his adopted bat stopped me in my tracks. It showed a bat with a scorpion in its mouth! Bats eat scorpions? How is that possible? Bats eat flying insects, right?

After reading the scientific literature on Antrozous pallidus and looking at the Bat Conservation International website, here’s the pallid bat’s story. It’s far weirder than I imagined.

Pallid bats live in the western United States and are the size of a sparrow but with the wingspan of a crow—though they are of course mammals and not birds.

Their big wings allow them to hover and catching their food on the ground. They have large ears that can hear the footsteps of a scorpion or centipede (another favorite food).

The bats appear to be immune to scorpion stings from even the most venomous scorpion in North America, the Arizona bark scorpion. Up to 70% of a pallid bat’s diet can be scorpions at certain times of the year.

But how does a bat find a scorpion at night on the ground given that the sounds from a scorpion’s feet are rather minimal?

Like many small bats, pallid bats use echolocation, a kind of bio sonar, to “see” in the dark. But weirdly, pallid bats stop using their sonar when they attack prey on the ground.

So what do they use to guide them in for a pinpoint pickup of a fleet-footed scorpion? Is it just their sensitive hearing, or might it also be their eyes?

Pallid bats have unusually large eyes for a bat, and several species of bats are known to be able to see ultraviolet light. Might the pallid bat also see ultraviolet light? My hypothesis is that they do. Scorpions, and to a less extent centipedes, fluoresce brightly under a ‘black’ light.

So pallid bats might use ultraviolet light to zero-in on dinner. But this presents an evolutionary problem. If pallid bats spot scorpions by their florescence, why has this characteristic not been deselected in scorpions?

After all, the scorpions get eaten if they are caught by the bat. The fluorescence appears to allow scorpions to find shelter quickly. A recent study found that the body of a scorpion is like a giant proto-eye that can sense ultraviolet shadows even in the dark. It’s what helps them find any nearby hiding place quickly and save themselves from a predator. Perhaps it’s a scorpion’s fluorescence helps against more numerous enemies than a few super-powered bats.

When the weirdness is added together what we have is a scorpion-eating flying mammal that’s immune to scorpion stings, can hear the rustle of a scorpion from a distance, can hover above the ground, has echo location but doesn’t use it when attacking scorpions, and may be able to see the ultraviolet light that scorpions reflect. Bats are amazing creatures.

Craig Leisher is a Senior Social Scientist who focuses on amplifying and measuring the benefits to people from conservation initiatives.

Comments: Weird Nature: A Bat that Eats Scorpions

Comment from Gary P Bell

August 13, 2013 at 12:05 pm

There is an important concept in science known as “Occam’s Razor”, or the “Principle of Parsimony” which states that given competing hypotheses the one which depends upon the fewest assumptions should be selected.
Given that; (1) Pallid Bats make no particular specialization on scorpions but sometimes include them among their very, very broad range of prey, (2) Pallids are able to find their terrestrial prey quite handily using the sounds the prey make, (3) they will drop off an attack if the prey stops moving, and (4) there is no evidence to suggest that Pallid Bats have some unique adaptation to their all-rod cell retinas to be able to see in the ultraviolet, this “phosphorescent scorpion” idea just doesn’t hold water.
Pallid Bats have pretty good visual acuity at low light levels, binocularity, and exceptional hearing over a broad range of frequencies, including lower frequencies, in addition to their fairly “standard equipment” echolocation system which is not particularly specialized for detecting prey on surfaces. They depend mostly on prey sounds and vision to support their unorthodox hunting style.
You can find my original paper on Pallid Bat sensory modality in the Behavioral Ecology and Sociobiology
June 1982, Volume 10, Issue 3, pp 217-223; “Behavioral and ecological aspects of gleaning by a desert insectivorous bat Antrozous pallidus (Chiroptera: Vespertilionidae)”.

Your 1982 paper was my first read when I searched the literature on Pallid bats. I’m sure you’re right and Pallid bats find their prey with their acute hearing and vision, but when I read about other species of bats with the ability to see ultraviolet light in Nature (Winter et al. 2003) and PNAS (Zhao et al. 2009) and recalled how scorpions and centipedes’ florescence under an ultraviolet light, I wondered if anyone had studied Pallid bats’ eyes. Another paper you authored notes that Pallid bats retain their visual acuity even at light levels “comparable to ground luminance on a clear, moonless night”, but I found no studies of the sensitivity to different wavelengths of light in Pallid bats. I was intrigued by the Zhao et al. (2009) paper that studied 33 species of bats (but not Pallid bats) and suggested “UV color vision plays a considerably more important role in nocturnal mammalian sensory ecology than previously appreciated.” Hence my hypothesis.

Great information on UV sensitivity in bats. I was not aware of those – thanks for the references. One thing to note in those studies is that there are very few cone cells in the retinas of those species of bats in which they have been found – most of the receptors (97% or more) are made of of densely-packed rod cells, up to 400,000 per mm2. I suspect that if some bats have cone cells that are sensitive to UV the function may have something to do with circadian clock-setting or perhaps azimuthal orientation rather than strictly visual.

A couple of other important points of clarification; (1) Pallid bats do not have very acute or sensitive vision compared with other Microchiropteran bats. My lab tests with Antrozous showed it to be a pretty mediocre visual performer, actually less visually-inclined than the Big Brown Bat, Eptesicus fuscus, although they do have slightly greater binocular overlap. The species I referred to that has high visual acuity even at low light levels is the California Leaf-nosed bat, Macrotus californicus (now THERE is a remarkable creature!). And Macrotus has twice the binocular overlap of the Pallid bat suggesting the ability to use vision for depth perception.

(2) I would use great caution in interpreting behavioral responses of animals to Black Lights. Although these instruments do emit the greatest amount of energy in the UV band, they still emit light in the visual spectrum. Some bulbs are filtered to minimize visual wavelengths and some are not, but even the filtered ones emit visible spectrum.

(3) But the other problem is that the UV issue is really moot because scorpions aren’t emitting UV, they are “simply” fluorescing in response to UV light. So, if Pallid bats were in any way visually specialized to see scorpions it should be in the Blue-Green and not in the Ultraviolet. But where Pallid’s are specialized is in their low-frequency hearing sensitivity, as would be expected.

Interesting article, this is another great example of the diversity amongst bats. Whether its habitat, diet, ability its simply amazing. Working in humane bat removal and bat control in the Columbus, Cincinnati and Dayton, Ohio area we often hear several myths and misconceptions about bats. I have a lot of great information about humane bat removal and bat control if anyone would like at http://www.propestmen.com/ohio/critters/bats/bat_removal_bat_control.html We also have a lot of information about bats in general at http://www.propestmen.com/ohio/index.html it may be very useful to some so feel free to take a look. Thanks again for the fascinating article, I really enjoyed it.

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